Low Power Apparatus and Method to Measure Complex Electrical Admittance or Impedance

ABSTRACT

An apparatus for measuring complex electrical admittance and/or complex electrical impedance in animal or human patients includes a first electrode and at least a second electrode which are adapted to be disposed in the patient. The apparatus includes a housing adapted to be disposed in the patient. The housing has disposed in it a stimulator in electrical communication with at least the first electrode to stimulate the first electrode with either current or voltage, a sensor in electrical communication with at least the second electrode to sense a response from the second electrode based on the stimulation of the first electrode, and a signal processor in electrical communication with the sensor to determine the complex electrical admittance or impedance of the patient.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 13/425,119filed Mar. 20, 2012, which claims the benefit of U.S. provisional patentapplication Ser. No. 61/516,138 filed Mar. 30, 2011, both of which areincorporated by reference herein.

FIELD OF THE INVENTION

The present invention is related to measuring complex electricaladmittance and/or complex electrical impedance in animal or humanpatients using low power. (As used herein, references to the “presentinvention” or “invention” relate to exemplary embodiments and notnecessarily to every embodiment encompassed by the appended claims.)More specifically, the present invention is related to measuring complexelectrical admittance and/or complex electrical impedance in animal orhuman patients using low power where the low power that is used is lessthan an average current of less than 23 mA in operation over time.

BACKGROUND OF THE INVENTION

This section is intended to introduce the reader to various aspects ofthe art that may be related to various aspects of the present invention.The following discussion is intended to provide information tofacilitate a better understanding of the present invention. Accordingly,it should be understood that statements in the following discussion areto be read in this light, and not as admissions of prior art.

Congestive heart failure (CHF) is one of the leading causes of admissionto the hospital [1]. Studies have shown that patients with dilatedhearts have a reduction in the frequency of hospital admission andprolongation of life with the implantation of bi-ventricular pacemakersand automatic implantable cardiac defibrillators [AICDs, 2-6]. Recently,“piggybacking” technology onto AICDs and bi-ventricular pacemakers forsensing the progression of impending CHF to reduce the number and lengthof stay of hospital admissions for CHF has been proposed [7-18]. Thereare two clinically tested “piggybacked” heart failure warning systemsplaced on bi-ventricular pacemakers and AICDs to reduce hospitaladmissions. First, Chronicle® measures right heart pressures in anattempt to monitor increases that are indicative of heart failure[11-13]. Second, Optivol® and CorVue® use lung conductance measurementsas an indication of pulmonary edema [8-10, 15]. However, both aredownstream measures of the earliest indicator of impending heartfailure—left ventricular (LV) preload or left ventricular end-diastolicvolume (LVEDV).

Conductance measurements have been available as an invasive tool todetect instantaneous LV volume since 1981 [25, 26]. Conductancetetrapolar electrodes are usually placed on a lead located within theheart chamber to determine instantaneous volume (FIG. 2). Conductancesystems generate an electric field (22) using a current source andvolume is determined from the returning voltage signal. Prior art showshow to separate the blood and muscle components from the combinedvoltage signal to determine LV preload from previously implanted AICDand bi-ventricular pacemakers.

Significant improvement in patient care could be achieved by adding theadmittance apparatus [19-24] to pacemakers and AICDs, using currentlydeployed bi-ventricular and AICD leads, to electrically detect eithertrue LV preload, or an increase in LV preload from baseline.Bi-ventricular and the RV AICD leads are already located in the ideallocations—the lateral LV epicardium and the right ventricular (RV)septum (FIGS. 1 a, 1 b). Since blood has 5-fold lower resistivity thanmyocardium, the preferential path (22) for a substantial fraction of thecurrent flow will be the LV blood volume. This low-power admittanceapparatus can be “piggy-backed” onto implanted AICD and bi-ventricularpacemakers to serve as an early warning system for impending heartfailure. Piggy-backed means one can take an existing pacemaker designand add this apparatus to it, without major redesign of the pacemakeritself. This means the admittance circuits need not be included in oneof the internal pacemaker chips; rather, it could be added to the systemwithout redesigning the pacemaker circuits themselves. This isparticularly true because much of this invention involves a change insoftware requiring only modest changes in hardware. particular, theapparatus uses the same leads, the same communication channel, and thesame power source as the pacemaker. The apparatus can be triggered bythe pacemaker or it can run untriggered (i.e., it runs periodically).The output of the apparatus will be a true/false warning signal, or aquantitative measure of heart volume. In this configuration, theapparatus does not alter how the pacemaker operates.

On the other hand, if one wished to design a new pacemaker, thisapparatus can also be used to dynamically adjust parameters in thepacemaker itself to maximize heart pumping efficiency. Furthermore, thevolume information could be used to improve the effectiveness ofventricular tachycardia detection in an automatic defibrillator.

A version of this apparatus can be implanted in animals (including, butnot limited to mice, rats, dogs, and pigs), which includes a pressurechannel and a wireless link (FIGS. 5 and 14). The lead is placed in aventricle (FIG. 2), and the experiment duration can vary from 1 day to 6months. The duration of the experiment is limited by the animal survivaland the storage capacity of the battery, and not the operation of theapparatus. The apparatus measures heart muscle function (leftventricular pressure-volume relationships) in these animals, and can beused for new drug discovery. These animals must be un-tethered andfreely roaming so that the transmitted signals are physiologic. Theapparatus transmits pressure-volume data (52), and a computer-basedreceiver collects, displays, and stores the data.

There are two papers describing a technique to detect heart failure[27-28] that may seem similar to the approach herein. In this technique,the current source and sink electrodes are both in the RV, and thesensing electrodes on the LV free wall. This means the electrical fieldwill be confined to the RV. So even though they have sensing electrodeson the LV wall, their signal will have a very weak dependence on theleft heart volume, detecting only fringing fields. Their type ofmeasurement is very noise prone, and this problem worsens as the heartenlarges because the septum blocks much of the field from reaching theLV free wall. It is believed that the present invention is superiorbecause 1) the majority of the sensing field goes across the blood poolin the left ventricle due to the relative conductivity of blood beinghigh, and source and sink electrodes being placed on opposite sides ofthe LV, and 2) because the heart muscle is removed as an artifact of themeasurement using admittance. Therefore, it is believed there are noavailable or proposed technologies that can perform chronic volumemeasurements of the left ventricle, including LVEDV, LVESV, and LVSV.

BRIEF SUMMARY OF THE INVENTION

The present invention pertains to a method for measuring complexelectrical admittance and/or complex electrical impedance in animal orhuman patients. The method comprises the steps of stimulating with astimulator disposed in a housing disposed in the patient with two ormore electrodes disposed in the patient with either current or voltage.There is the step of sensing with a sensor disposed in the housing withtwo or more sensing electrodes disposed in the patient to sense aresponse from the sensing electrodes based on the stimulation of thestimulating electrodes. There is the step of determining with a signalprocessor disposed in the housing and in electrical communication withboth the stimulator and the sensor the complex electrical admittanceand/or complex electrical impedance of the patient, the stimulator andthe sensor and the signal processor together using less than an averagecurrent of less than 23 mA in operation over time at a voltage less than3.7 V.

The present invention pertains to an apparatus for measuring complexelectrical admittance and/or complex electrical impedance in animal orhuman patients. The apparatus comprises a first electrode and at least asecond electrode which are adapted to be disposed in the patient. Theapparatus comprises a housing adapted to be disposed in the patient, thehousing having disposed in it a stimulator in electrical communicationwith at least the first electrode to stimulate the first electrode witheither current or voltage, a sensor in electrical communication with atleast the second electrode to sense a response from the second electrodebased on the stimulation of the first electrode, and a signal processorin electrical communication with the sensor to determine the complexelectrical admittance or impedance of the patient, the stimulator andthe sensor and the signal processor together using less than an averagecurrent of less than 23 mA in operation over time.

The present invention pertains to a method for measuring complexelectrical admittance and/or complex electrical impedance in animal orhuman patients. The method comprises the steps of stimulating with astimulator disposed in a housing disposed in the patient with at leasttwo stimulating electrodes disposed in the patient with either currentor voltage. There is the step of sensing with a sensor disposed in thehousing with at least two sensing electrodes disposed in the patient tosense a response from the sensing electrodes based on the stimulation ofthe simulating electrodes. There is the step of determining with asignal processor disposed in the housing and in electrical communicationwith the sensor the complex electrical admittance or impedance of thepatient, the stimulator and the sensor and the signal processor togetherusing less than an average current of less than 23 mA in operation overtime.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

In the accompanying drawings, the preferred embodiment of the inventionand preferred methods of practicing the invention are illustrated inwhich:

FIGS. 1a and 1b show four electrodes placed in or around the heart usingtwo or more leads.

FIG. 2 shows a single four-electrode lead placed in a ventricle.

FIG. 3 shows ADC input voltage showing the chirped sinusoid generated bythe SinDAC.

FIG. 4 is a block diagram of the current stimulation portion of theapparatus.

FIG. 5 is a block diagram of the voltage sensing portion of theapparatus, with optional pressure, and with optional wireless link.

FIG. 6 shows SinDAC used to create sine waves.

FIG. 7 shows measured outputs from 1-bit, 3-bit, and 7-bit SinDACimplementations.

FIGS. 8a and 8b show front and back photographs of a prototype circuit.

FIGS. 9a-9d show experimental data showing the signal to noise ratio isthe equivalent of 10-bits.

FIGS. 10a and 10b show experimental data showing the analog power supplyduring chirping.

FIGS. 11 a, 11 b and 11 c show experimental data showing the linearityin resistance (11 a) and conductance (11 b, 11 c).

FIGS. 12a and 12b show experimental data showing the linearity insusceptance.

FIG. 13 shows experimental data demonstrating how the apparatus canmeasure heart volume.

FIG. 14 is a photograph of the wireless version of the prototype.

FIG. 15 shows experimental data showing the wireless data as transmittedby the apparatus.

FIG. 16 is a block diagram of the apparatus using a synchronousdemodulator with four-quadrant mixers.

FIG. 17 shows voltage across 100 ohm R₂₋₃ resistor (CH2) and currentwaveforms (CH1) measured with the synchronous demodulator version.

FIG. 18 shows an electronic circuit that can be used to measure heartvolume.

FIG. 19a is a flow chart of software used to create output and sampleinput.

FIG. 19b is a flow chart of software used to measurements into volume.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein like reference numerals refer tosimilar or identical parts throughout the several views, and morespecifically to FIGS. 1 a, 1 b and 2 thereof, there is shown anapparatus 100 for measuring complex electrical admittance and/or complexelectrical impedance in animal or human patients. The apparatus 100comprises two or more electrodes 10 that are adapted to be disposed inthe patient. The apparatus 100 comprises a housing 110 adapted to bedisposed in the patient. The housing 110 has disposed in it a stimulator112 in electrical communication with two or more electrodes 10 tostimulate with either current or voltage, a sensor 114 in electricalcommunication with the same stimulating electrodes 10 or with additionalelectrodes 10 to sense a response based on the stimulation of thestimulating electrodes 10, and a signal processor 116 in electricalcommunication with both the stimulator 112 and the sensor 114 todetermine the complex electrical admittance or impedance of the patient.The stimulator 112 and the sensor 114 and the signal processor 116together use less than an average current of less than 23 mA inoperation over time.

The present invention pertains to a method for measuring complexelectrical admittance and/or complex electrical impedance in animal orhuman patients. The method comprises the steps of stimulating with astimulator 112 disposed in a housing 110 disposed in the patient withtwo or more electrodes 10 disposed in the patient with either current orvoltage. There is the step of sensing with a sensor 114 disposed in thehousing 110 with two or more sensing electrodes 10 disposed in thepatient to sense a response from the sensing electrodes 10 based on thestimulation of the stimulating electrodes 10. There is the step ofdetermining with a signal processor 116 disposed in the housing 110 andin electrical communication with both the stimulator 112 and the sensor114 the complex electrical admittance and/or complex electricalimpedance of the patient, the stimulator 112 and the sensor 114 and thesignal processor 116 together using less than an average current of lessthan 23 mA in operation over time at a voltage less than 3.7 V.

The present invention pertains to an apparatus 100 for measuring complexelectrical admittance and/or complex electrical impedance in animal orhuman patients. The apparatus 100 comprises a first electrode and atleast a second electrode which are adapted to be disposed in thepatient. The apparatus 100 comprises a housing 110 adapted to bedisposed in the patient, the housing 110 having disposed in it astimulator 112 in electrical communication with at least the firstelectrode to stimulate the first electrode with either current orvoltage, a sensor 114 in electrical communication with at least thesecond electrode to sense a response from the second electrode based onthe stimulation of the first electrode, and a signal processor 116 inelectrical communication with the sensor 114 to determine the complexelectrical admittance or impedance of the patient, the stimulator 112and the sensor 114 and the signal processor 116 together using less thanan average current of less than 23 mA in operation over time.

The present invention pertains to a method for measuring complexelectrical admittance and/or complex electrical impedance in animal orhuman patients. The method comprises the steps of stimulating with astimulator 112 disposed in a housing 110 disposed in the patient with atleast two stimulating electrodes 10 disposed in the patient with eithercurrent or voltage. There is the step of sensing with a sensor 114disposed in the housing 110 with at least two sensing electrodes 10disposed in the patient to sense a response from the sensing electrodes10 based on the stimulation of the simulating electrodes 10. There isthe step of determining with a signal processor 116 disposed in thehousing 110 and in electrical communication with the sensor 114 thecomplex electrical admittance or impedance of the patient, thestimulator 112 and the sensor 114 and the signal processor 116 togetherusing less than an average current of less than 23 mA in operation overtime.

The present invention pertains to an apparatus 100 for measuring complexelectrical admittance and/or complex electrical impedance in animal orhuman patients. The apparatus 100 comprises two or more electrodes 10that are adapted to be disposed in the patient. The apparatus 100comprises a housing 110 adapted to be disposed in the patient. Thehousing 110 has disposed in it a stimulator 112 in electricalcommunication with two or more electrodes 10 to stimulate with eithercurrent or voltage, a sensor 114 in electrical communication with thesame stimulating electrodes 10 or with additional electrodes 10 to sensea response based on the stimulation of the stimulating electrodes 10,and a signal processor 116 in electrical communication with both thestimulator 112 and the sensor 114 to determine the complex electricaladmittance or impedance of the patient. The stimulator 112 and thesensor 114 and the signal processor 116 together use less than anaverage current of less than 23 mA in operation over time.

The signal processor 116 may measure a real part, an imaginary part, amagnitude, and/or phase of admittance. The signal processor 116 maymeasure a real part, an imaginary part, a magnitude, and/or phase ofimpedance can be measured. The stimulator 112 may produce an excitationwave that is a sinusoid at a single frequency, greater than 0 and lessthan or equal to 1 MHz. The stimulator 112 may produce an excitationwave that is two or more sinusoids with frequencies greater than 0 andless than or equal to 1 MHz. The stimulator 112 may produce anexcitation wave that is any shape that can be defined by a repeatedsequence of integer values, whose frequency components range from 0 to 1MHz.

The stimulator 112 may produce an excitation wave that is created by aresistor-summing network, called a SinDAC, such that a number ofresistors, resistor values, digital output sequence, and rate of digitaloutputs are selected to define a shape and frequency of the excitationwave. An ADC conversion of the sensor 114 may be synchronized to theSinDAC outputs generating the stimulation. A Discrete Fourier Transform(DFT) may be used by the signal processor 116 to extract complexelectrical properties.

The complex measurements may occur with an analog circuit using asynchronous demodulator to directly measure either impedance oradmittance. While measuring complex electrical properties 100 times asecond may require less than 500 μA of current. While measuring complexelectrical properties 50 times an hour may require less than 1 μA ofcurrent.

The size of the housing 110 may be less than 2 cm by 2 cm by 0.4 cm. Theelectrodes 10 may be placed in or on the heart, which are used toestimate heart volume, stroke volume, change in heart volume, and/orchange in stroke volume of the patient. The apparatus 100 may include apressure sensor 114 disposed on the lead, which is used to measurepressure volume loops in the heart. The apparatus 100 may include awireless link and recording base station 118 which are used to remotelymeasure pressure, heart volume, stroke volume, change in heart volume,and/or change in stroke volume of the patient. The housing 110 mayinclude a pacemaker.

In the operation of the invention, a low-power method and apparatus 100has been designed to measure electrical impedance and electricaladmittance in, on, and across the heart. Electrical impedance (Z) is theratio of the effort divided by flow as electrical energy flows throughan object. Electrical admittance (Y) is the ratio of flow divided byeffort. The impedance and admittance of living tissue are complexnumbers; this means electrical energy is both reduced in amplitude anddelayed in time (phase shift) by the tissue during transfer. Theelectrical measurements can be used to determine heart volume, change inheart volume, and/or stroke volume. Prior art has defined the lead andthe relationship between electrical properties and heart physiology.Herein is presented in substantial detail several possible embodimentsof a method and apparatus 100, used to measure the electricalproperties, that is both low in power and small in size. Othertechnically-similar embodiments using the same overall power-savingstrategies will likely prove equally effective in making thesemeasurements.

The apparatus 100 can be used in telemetric applications of heart musclefunction in animals. There is a desire to develop a low power, smallsized, implantable systems to last up to six months. Using the low-powerstrategy that is described herein will make it possible to implant theapparatus 100 in animals to study the long term effect of drug therapiesfor treating cardiac diseases. For instance, it can be used ingene-altered mouse hearts for new drug discovery. Animals with theseimplantable apparatuses would be un-tethered, and freely roaming.

More importantly, the apparatus 100 can be incorporated into existingpacemakers and used to detect early-stage congestive heart failure.Given the apparatus 100, the doctors will be able to adjust drug dosesand prevent costly hospital visits.

The apparatus 100 could also be used in an adaptive pacemaker to adjustthe timing of the electrical stimulations in order to maximize heartpumping efficiency.

In summary, a technique and an apparatus 100 has been developed, whichis both low power and small size, capable of measuring heart volume,change in heart volume, stroke volume, and/or change in stroke volume.

10—Four Electrodes for Admittance Measurement, Located in the Ventricle

One example of an existing four-electrode lead is the ScisenseFTE1-1912B-8018. This is a 1.9 F Pressure-Volume Lead. This is aflexible and soft rat pressure-volume lead with an 8 mm ring spacing foruse with average sized rats. The diameter is 1.9 F and the distance fromelectrodes 1,2 to electrodes 3,4 is about 10 mm. In addition, theelectrodes on existing leads of existing products (see description ofpacemaker 14) can be used with humans.

12—A Single Lead Placed in the Ventricle

FIGS. 1a and 1b show the lead (10) in the right ventricle.

14—The Present Invention can be Added to or Embedded into ExistingProducts

Two examples of existing products (pacemakers) into which the apparatus100 could be embedded is Optivol® (Medtronic, Minneapolis, Minn.) andCorVue® (St. Jude Medical, St. Paul, Minn.). When embedding theapparatus 100 into an existing apparatus, the existing apparatus willinclude a housing 110 and power supply. The Medtronic InSync® ICD model7272 housing 110 is box-shaped with 8 mm radius curves on all edges. Theoutside dimensions of its metal case are 57 mm wide, 72 mm tall, 16 mmtall. The metal shell is 1 mm thick and is made from a titanium alloy.There is a plastic connector on top with dimensions 35 mm wide, 20 mmtall, 15 mm wide. Over 50% of the internal space is occupied by thelithium battery. Many pacemakers use lithium/iodine-polyvinylpyridineprimary batteries, which can last 8 to 10 years. The housing 110 of theapparatus 100 can be smaller since the components of the invention canbe smaller. The size without battery and lead can be smaller than 11 mL,such as 9 mL or 7 mL or 5 mL. The weight can be less than 9 g, such as 7g or 5 g or 4 g. The average power=current*3.6V to operate the sensor114, stimulator 112 and signal processor 116 can be less than 86 mW,such as 75 mW or 65 mW or 55 mW or 35 mW. The average current to operatethe sensor 114, stimulator 112 and signal processor 116 can be less than24 mA, such as 20 mA or 17 mA or 14 mA. The current while sampling withthe sensor 114, stimulator 112 and signal processor 116 can be less than42 mA, such as 35 mA or 28 mA or 22 mA. See table 1 below whichidentifies these various properties in terms of what has been actuallybuilt and is operative for the technique described herein.

16—The System can Communicate with the Patient or Medical Staff

Existing wireless protocols such as SimpliciTI™ by Texas Instruments areused for wireless communication.

18—Electrodes 1 and 2 Placed on the Heart (in a Coronary Vein)

One example of an existing lead that can be positioned in the coronaryvein is the St. Jude Quicksite XL 1058T. It is 75.86 cm long. It has adiameter of 5.0 F at the distal lead and a diameter of the 5.6 F at theproximal lead. The lead (18) is positioned into the coronary vein duringimplantation surgery. The veins are on the epicardial surface of theheart. The position of electrodes 1,2 of the lead will be fixed relativeto the position of the vein by the scarring occurring at the insertionsite of the lead where it enters the vein. In other words, electrodes 1,2 will be at a fixed position on the epicardial surface of the heart.Because the heart is beating, electrodes 1, 2 will move relative toelectrodes 3, 4.

20—Electrodes 3 and 4 Placed in the Heart (in a Ventricle or Atrium)

One example of an existing lead that can be positioned in the rightventricle is the St Jude Tendril SDX 1688TC. This lead is bipolar, canbe used in the atria or ventricle. It has a screw-in electrode. It comesin lengths of 34, 40, 46, 52, 58, 85, 100 cm. It uses a 7 F introducer.One example of an existing lead that can be positioned in the rightatrium is the St Jude Optisense 1699TC. This is a pacing bipolarelectrode and is also 7 F in diameter. It comes in lengths of 40 46 and52 cm.

The lead (20) is positioned into either the apex of the right ventricleor into the right atrium. This lead is inserted via the systemic veinsand subsequently screwed into the myocardial tissue. Therefore,electrodes 3 and 4 are fixed on the endocardial surface of either theright ventricle or right atrium. The distance between electrode pair 1,2 and pair 3, 4 will be about 70 to 100 cm, and will vary as the heartbeats. When using lead (10), the distance between 1, 2 and 3, 4 isfixed. When using lead (18) and (20), the distance between 1, 2 and 3, 4is variable. This variable distance is incorporated into the equationused to convert blood resistance to volume (92).

When using lead 10, the distance between 1, 2 and 3, 4 is fixed. Whenusing lead 18 and 20, the distance between 1, 2 and 3, 4 is variable.This variable distance is incorporated into the equation used to convertblood resistance to volume (92).

24—Microcontroller or Digital Logic

One example of an existing microcontroller that can be used with theapparatus 100 is the Texas Instruments MSP430F2013. This microcontrollerhas 2048 bytes of Flash EEPROM, 128 bytes of RAM, and runs at 16 MHz. Itcan be used to measure volume, and comes in a 16-pin surface mountpackage occupying about 4 by 4 by 1 mm.

41—Pressure Sensor (Optional)

One example of an existing pressure sensor is the one included on theScisense FTE1-1912B-8018 lead. This is a 1.9 F Pressure-Volume Lead.

42—Low Power Amplifier and 43—Low Power Amplifier for the PressureChannel

One example of a low power amplifier is the Texas Instruments INA322.This instrumentation amp runs with 490 μA of supply current, and hasbandwidth of 2 MHz at a gain of 25.

56—Antenna Used to Send Wireless Communication

One example of an antenna is the ANT-916-CHP antenna from AntennaFactor. It is a surface mount part that operates at 916 MHz.

84—Discrete Fourier Transform

The Discrete Fourier Transform converts signals in the time domain intothe frequency domain.

The four-electrode leads and there placement, shown in FIG. 1 a, FIG. 1band FIG. 2 are well known to one skilled in the art. The four electrodes(32, 34, 36, 38, shown in FIGS. 4 and 5) are either placed around theheart as shown in FIG. 1a and FIG. 1 b, or in the heart as shown in FIG.2. A sinusoidal current is applied to electrodes 1 (32) and 4 (38), andthe resulting voltage is measured between electrodes 2 (34) and 3 (36).Although the description refers to a four-electrode configuration, thetechnique here will also operate with any configuration using 2 or moreelectrodes. If the number of electrodes is less than four, then eitheror both electrode pairs 1-2 or 3-4 are shared. When using more than 4electrodes, two electrodes are used to supply the sinusoidal current,and the remaining electrodes are used in pairs to measure the volumebetween the electrode pairs. The methods that convert electricalmeasurements into heart volumes are also well known to one skilled inthe art. The method to remove the muscle component from the conductanceand admittance signals is well known to one skilled in the art. (Seepatents and applications in reference list below). This presentinvention focuses on conversion to practice that is both small in sizeand extremely low in power.

The fundamental theory uses the Discrete Fourier Transform (DFT). Theinput to the DFT will be N samples versus time, and the output will be Npoints in the frequency domain. The sampling rate is defined as f_(s).

Input: {x _(n) }={x ₀ , x ₁ , x ₂ , . . . , x _(N−1)}

Output: {X _(k) }={X ₀ , X ₁ , X ₂ , . . . , X _(N−1)}

The definition of the DFT is

${X_{k} = {{\sum\limits_{n = 0}^{N - 1}\; {x_{n}W_{N}^{kn}\mspace{14mu} {where}\mspace{14mu} W_{N}}} = {{^{{- {j2\pi}}/N}\mspace{14mu} k} = 0}}},1,2,\ldots \mspace{11mu},{N - 1}$

The DFT output X_(k) at index k represents the amplitude and phase ofthe input at frequency k*f_(S)/N (in Hz). The SinDAC output and the ADCinput occur at f_(s). An ADC sample occurs every time T=1/f_(s). If M isthe number of SinDAC outputs per wave, then the sinusoidal frequency fis f_(s)/M. The DFT resolution in Hz/bin is the reciprocal of the totaltime spent gathering time samples; i.e., 1/(N*T)=f_(s)/N. To measure thevoltage response at frequency f, set f_(s)/M=k*f_(S)/N, and look at justthe one term, k=N/M. Recall that both M=2^(m) and N=2^(n) are powers of2, where m and n are integers with n greater than or equal to m. Thismeans k will also be a power of 2, which will greatly simplify thecalculation of the DFT for the one point at k. The number of periods inthe sample space will be N/M. Since the excitation is constant current,the calculated voltage response is a measure of impedance. Although, itwill be lower power to set N/M to be a power of 2, the method will workfor any integers {N, M} such that N/M is an integer.

Three specific illustrative examples are given, but the method will workfor any integers {N, M} such that N/M is an integer. In the firstexample, let f=5 kHz, M=8, and N=16. FIG. 4 shows a block diagram of theconstant current output circuit. The microcontroller (24) writes digitalvalues (26) to the SinDAC (28) at a rate of f_(s)=M*f=40 kHz. With f_(s)equal to 40 kHz, the time between samples is Δt=25 μs. The circuit (30)drives lead pins 1 (32) and 4 (38) with an AC current of constantamplitude and a frequency of 5 kHz. FIG. 5 shows a block diagram of thevoltage sensing input circuit. The differential voltage across lead pins2 (34) and 3 (36) is amplified (42) and filtered (44). The filter (44)is optional, meaning the apparatus 100 will function without it,reducing power at the expense of reduced signal/noise ratio. Theresulting signal is sampled by the ADC (46) at the same f_(s)=40 kHzrate. A total of N=16 data points (two periods) are collected with atotal sample time=400 μs. The input sampling rate is synchronized to theoutput rate of the two SinDACs. Let the sampled inputs be x₀, x₁, x₂, .. . , x₁₅. Since the sampling rate is 40 kHz, the k=2 term represents 5kHz. In other words, X₂ represents complex impedance at f=5 kHz. For anN=16 DFT, calculate the complex constants:

W ^(k)=exp(−2πik/16)=cos(2πk/16)−i*sin(2πk/16)

If M and N are powers of two, the DFT term at k=N/M will be very simpleto calculate—this is the essence of the well-known Fast FourierTransform (FFT) algorithm, which is presently in wide-spread use. Inthis first example, to calculate the k=2 term, only every other W^(k)term is needed:

ReZ=Re[Z ₂ ]=x ₀ −x ₄ +x ₈ −x ₁₂+√1/2*(x ₁ −x ₃ −x ₅ +x ₇ +x ₉ −x ₁₁ −x₁₃ +x ₁₅)

ImZ=Im[Z ₂ ]=−x ₂ +x ₆ −x ₁₀ +x ₁₄+√1/2*(−x ₁ −x ₃ +x ₅ +x ₇ −x ₉ −x ₁₁+x ₁₃ +x ₁₅)

Prior implementations measured magnitude and phase, and then calculatedthe real and imaginary parts using trigonometric functions. It is notneeded to calculate magnitude and phase. However, if desired magnitudeand phase could be calculated as

Mag|Z ₂|=sqrt(Re[Z ₂ ]*Re[Z ₂ ]+Im[Z ₂ ]*Im[Z ₂])

Angle(Z ₂)=arctan(Im[Z ₂ ]/Re[Z ₂])

√1/2 can be approximated as a fixed-point number with sufficientaccuracy. Because the input to the lead system is constant current, theoutput is the real and imaginary part of the impedance at 5 kHz. Let Zbe the complex impedance for the k=2 term

Z=ReZ+jImZ

One possible fixed-point implementation (48) is

ReZ=(17*(x ₀ −x ₄ +x ₈ −x ₁₂)+12*(x ₁ −x ₃ −x ₅ +x ₇ +x ₉ −x ₁₁ −x ₁₃ +x₁₅))/16

ImZ=(17*(−x ₂ +x ₆ −x ₁₀ +x ₁₄)+12*(−x ₁ −x ₃ +x ₅ +x ₇ −x ₉ −x ₁₁ +x ₁₃+x ₁₅))/16

The “16” in the above equations is arbitrary because the apparatus 100will be calibrated. As an example of a low-power multiply, consider thesimple case of 17*x. The “multiply by 17” is rewritten as a “multiply by16” plus the addition of the input. In this way, the algorithm can beimplemented on a low-power microcontroller, as shown in the followingpseudo-code. These three steps require one store, 4 shifts and 1addition.

1) Set CopyOfX equal to x

2) Shift x left 4 times

3) Add CopyOfX to x

As a second example, let f=20 kHz, M=8, and N=32. The microcontroller(24) writes digital values (26) to the SinDAC (28) at a rate off_(s)=M*f=160 kHz. With f_(s) equal to 160 kHz, the time between samplesis Δt=6.25 μs. The circuit (30) drives lead pins 1 (32) and 4 (38) witha 20 kHz AC current. The voltage signal is sampled by the ADC (46) atthe same f_(s)=160 kHz rate. In this particular example a total of N=32data points are collected with a total sample time=200 μs—in fact, anyconvenient multiple of 2 can be used, so N=32 is just for illustration.Let the sampled inputs be x₀, x₁, x₂, . . . , x₃₁. Since the samplingrate is 160 kHz, the k=4 term represents the desired 20 kHz. In otherwords, X₄ represents complex impedance at f=20 kHz. For a 32-point DFT,calculate the complex constants:

W ^(k)=exp(−2πik/32)=cos(2πk/32)−i*sin(2πk/32)

The k=4 represents f=20 kHz. To calculate the k=4 term, only everyfourth W^(k) term is needed:

ReZ=Re[Z ₄ ]=x ₀ −x ₄ +x ₈ −x ₁₂ +x ₁₆ −x ₂₀ +x ₂₄ −x ₂₈+√1/2*(x ₁ −x ₃−x ₅ +x ₇ +x ₉ −x ₁₁ −x ₁₃ +x ₁₅ +x ₁₇ −x ₁₉ −x ₂₁ +x ₂₃ +x ₂₅ −x ₂₇ −x₂₉ +x ₃₁)

ImZ=Im[Z ₄ ]=−x ₂ +x ₆ −x ₁₀ +x ₁₄ −x ₁₈ +x ₂₂ −x ₂₆ +x ₃₀+√1/2*(−x₁ −x₃ +x ₅ +x ₇ −x ₉ −x ₁₁ +x ₁₃ +x ₁₅ −x ₁₇ −x ₁₉ +x ₂₁ +x ₂₃ −x ₂₅ −x ₂₇+x ₂₉ +x ₃₁)

Again, it is not need here to calculate magnitude and phase. Othersystems measure magnitude, |Z|, with or without phase angle, φ. Theseolder systems then use trigonometry to determine the real and imaginarypart of the signal. The following equations are presented only as acomparison to prior art.

ReZ=|Z|*cos(φ)

ImZ=|Z|*sin(φ)

Again, W^(k) terms can be approximated as a fixed-point numbers. Noticehow close 12/17 is to the √1/2 (0.70588 versus 0.70711). Usingfixed-point saves power. Let Z be the complex impedance for the k=4term. One possible fixed-point implementation (48) is

ReZ=(17*(x ₀ −x ₄ +x ₈ −x ₁₂ +x ₁₆ −x ₂₀ +x ₂₄ −x ₂₈)+12*(x ₁ −x ₃ −x ₅+x ₇ +x ₉ −x ₁₁ −x ₁₃ +x ₁₅ +x ₁₇ −x ₁₉ −x ₂₁ +x ₂₃ +x ₂₅ −x ₂₇ −x ₂₉ +x₃₁))/32

ImZ=(17*(−x ₂ +x ₆ −x ₁₀ +x ₁₄ −x ₁₈ +x ₂₂ −x ₂₆ +x ₃₀)+12*(−x ₁ −x ₃ +x₅ +x ₇ −x ₉ −x ₁₁ +x ₁₃ +x ₁₅ −x ₁₇ −x ₁₉ +x ₂₁ +x ₂₃ −x ₂₅ −x ₂₇ +x ₂₉+x ₃₁))/32

The divide by 32, implemented as a right shift, was added to adjust theamplitude of the calculation. Because the apparatus 100 will becalibrated, the “32” in these equations is arbitrary.

As a third example, let f=10 kHz, M=12, and N=24. The microcontroller(24) writes digital values (26) to the SinDAC (28) at a rate off_(s)=M*f=120 kHz. With f_(s) equal to 120 kHz, the time between samplesis Δt=8.33 μs. The circuit (30) drives lead pins 1 (32) and 4 (38) witha 10 kHz AC current. The voltage signal is sampled by the ADC (46) atthe same f_(s)=120 kHz rate. In this particular example total of N=24data points are collected with a total sample time=200 μs—in fact, anyk=N/M equal to an integer will work, so N=24 is just for illustration.Let the sampled inputs be x₀, x₁, x₂, . . . , x₂₃. Since the samplingrate is 120 kHz, the k=2 term represents the desired 10 kHz. In otherwords, X₂ represents complex impedance at f=10 kHz. For a 24-point DFT,calculate the complex constants:

W ^(k)=exp(−2πik/24)=cos(2πk/24)−i*sin(2πk/24)

The k=2 represents f=10 kHz. Notice that cos(π/6)=√3/4=0.8660. Tocalculate the k=2 term, only need every second W^(k) term is needed:

Re[Z₂ ]=x ₀ −x ₆ +x ₁₂ −x ₁₈+√3/4*(x ₁ −x ₅ −x ₇ +x ₁₁ +x ₁₃ −x ₁₇ −x ₁₉+x ₂₃)+1/2*(x ₂ −x ₄ −x ₈ +x ₁₀ +x ₁₄ −x ₁₆ −x ₂₀ +x ₂₂)

Im[Z ₂ ]=−x ₃ +x ₉ −x ₁₅ +x ₂₁+1/2*(−x ₁ −x ₅ +x ₇ +x ₁₁ −x ₁₃ −x ₁₇ +x₁₉ +x ₂₃)+√3/4*(−x ₂ −x ₄ +x ₈ +x ₁₀ −x ₁₄ −x ₁₆ +x ₂₀ +x ₂₂)

These equations can also be implemented in fixed-point math. Notice howclose 13/15 is to the √3/4 (0.8667 versus 0.8660). Using fixed-pointsaves power. Let Z be the complex impedance for the k=2 term. The “32”in the following equations is arbitrary because the apparatus 100 willbe calibrated:

Re[Z ₂]=((30*(x ₀ −x ₆ +x ₁₂ −x ₁₈)+26*(x ₁ −x ₅ −x ₇ +x ₁₁ +x ₁₃ −x ₁₇−x ₁₉ +x ₂₃)+15*(x ₂ −x ₄ −x ₈ +x ₁₀ +x ₁₄ −x ₁₆ −x ₂₀ +x ₂₂))/32

Im[Z ₂]=((30*(−x ₃ +x ₉ −x ₁₅ +x ₂₁)+15*(−x ₁ −x ₅ +x ₇ +x ₁₁ −x ₁₃ −x₁₇ +x ₁₉ +x ₂₃)+26*(−x ₂ −x ₄ +x ₈ +x ₁₀ −x ₁₄ −x ₁₆ +x ₂₀ +x ₂₂))/32

An important consequence of the M-to-1 ratio in both the SinDAC and theDFT is that the sampling frequency (f_(s)) need not be accurate. If theinput/output sampling rate is either a little too fast or too slow, thesystem still works. For example, if the sampling frequency drops by 5%,going from 160 kHz to 152 kHz, the only consequence is now theelectrical impedance and admittance measurements are being made at 19kHz instead of 20 kHz. The electrical properties of blood and tissue donot significantly vary for frequencies 19 to 21 kHz, so a 5% error inthe clock frequency will not affect the ability of the apparatus 100 tomeasure heart volume. It requires a significant amount of electricalpower to create a precise sampling clock. Conversely, this apparatus 100can derive its timing from a low-power voltage-controlled oscillator(VCO).

The basic idea of the SinDAC (28) is shown in FIG. 6. There are one ormore digital outputs from the microcontroller (26). Each output will be0 or +V volts, where +V is the V_(OH) of the CMOS electronics. Eachdigital output is connected a resistor. The resistor-summing networkcreates the voltage output of the SinDAC (V_(out)). The resistancevalues and digital output patterns are selected to match the V_(out)output to the desired sine wave. A capacitor can be added to smooth outthe voltage output, but good results can be obtained without thecapacitor shown in FIG. 6. FIG. 7 shows data measured with three SinDACimplementations, all of which create 20-kHz sinusoids. Let f be thedesired sine wave frequency. The 1-bit output pattern is {0, 1}occurring at a rate of 2*f kHz. One possible 8-element output patternfor the 3-bit SinDAC is {0, 1, 3, 7, 7, 6, 4, 0}. Since there are 8elements in this pattern, the SinDAC output rate should be 8*f. Onepossible 12-element output pattern for the 5-bit SinDAC is {0, 1, 3, 7,15, 31, 31, 30, 28, 24, 16, 0}, occurring at a rate of 12*f. Onepossible 16-element output pattern for the 7-bit SinDAC is {0, 1, 3, 7,15, 31, 63, 127, 127, 126, 124, 120, 112, 96, 64, 0} occurring at a rateof 16*f. These patterns are specific examples of a general approach withthe following five general characteristics:

1) The binary patterns have symmetry, because the sine wave issymmetric. The example patterns listed above were derived from a JohnsonCounter, which is an example of a ring counter [29]. In particular, thefollowing n-bit patterns were created using the top n bits of an(n+1)-bit Johnson counter. These patterns include, but are not limitedto the following:

1-bit 0, 1

3-bit 000, 001, 011, 111, 111, 110, 100, 000

5-bit 00000, 00001, 00011, 00111, 01111, 11111, 11111, 11110, 11100,11000, 10000, 00000

7-bit 0000000, 0000001, 0000011, 0000111, 0001111, 0011111, 0111111,

1111111, 1111111, 1111110, 1111100, 1111000, 1110000, 1100000, 1000000,0000000

The pattern need not be derived from a Johnson counter. For example,these patterns all create 8-element sequences. Any of these patternscould be used to create a sine wave that is 8 times slower than theoutput rate. I.e., M=8, or f_(s)=8*f.

3-bit 000, 001, 011, 111, 111, 011, 001, 000

3-bit 000, 001, 011, 110, 111, 011, 011, 001

4-bit 0000, 0001, 0011, 0111, 1111, 0111, 0011, 0001

5-bit 00000, 00001, 00111, 01111, 11111, 01111, 00011, 00001

6-bit 000000, 000001, 000111, 011111, 111111, 001111, 000011, 000001

2) The length of the pattern is much shorter than a pattern used by aregular DAC when creating a sine wave.

3) The number of bits is much smaller than an equivalent system using alinear DAC to create a sine wave.

4) The output rate is synchronized with the input rate.

5) A resistor summing circuit converts the binary pattern to a voltage.The individual resistor values determine the weight of each bit. Theweighting of each bit is neither equal nor a power of 2. Rather, theresistor values in FIG. 6 are found by minimizing the mean squared errorbetween the desired wave and actual wave using multidimensionalminimization techniques. An analog circuit (30) will convert the voltageoutput to a current.

The software can be adjusted to select the length of the chirp. FIG. 3shows the voltage measured from a 20 kHz 12-period chirp lasting 500 μs,generated by a 3-bit SinDAC. The chirped measurement will occur eitherperiodically or using a trigger input. For example, the apparatus 100could output 50 chirps at 10 Hz once an hour. Taking 50 chirps allowsthe apparatus 100 to find EDV throughout the cardiac and respiratorycycles. Performing measurements once an hour filters out the positiondependence. If there is an ECG trigger at end diastole, then the 50chirps at 10 Hz can be reduced to 5 chirps every heartbeat.

A prototype was built and calibrated as shown in FIGS. 8a and 8 b, whichimplements a 3-bit SinDAC and a 10-bit ADC. FIGS. 9a-9d show the FFTmeasurements on this system during a continuous sine wave output (notchirped). Signals at 140 kHz and 180 kHz will alias into the 20 kHz bincausing error. Actually, any signal with a frequency of n*160±20 kHz forany integer n will alias, but the 140 and 180 kHz will be the largestsource of error. Signal to noise ratio is defined as the ratio of the 20kHz signal to the 140 kHz and 180 kHz noises. Signals at otherfrequencies will be removed when calculating the single term of theN-point DFT. The ADC range is 0 to 2.5V, the resolution is 2.5 mV. A10-bit ADC is the equivalent of 20*log(1/1024)=−60 dB. These datademonstrate the prototype apparatus 100 has a signal to noise ratio ofabout 10 bits.

When the apparatus 100 is being used to detect heart failure (FIGS. 1 a,1 b), the complex impedance (Z=ReZ+jImZ) is sufficient. When theapparatus 100 is being used to measure volume with a lead placed in theheart (FIG. 2), admittance is required. Let Y be the complex admittance(1/Z) at 20 kHz.

Y=ReY+jImY=1/(ReZ+jImZ)=(ReZ−jImZ)/(ReZ ² +ImZ ²)

ReY=ReZ/(ReZ ² +ImZ ²)

ImY=−ImX/(ReX ² +ImZ ²)

Calculating the value ReZ*ReZ+ImZ*ImZ requires two multiplications,which can be implemented using shift and add. To prevent overflow withfinite precision math, the amplitude will be reduced. For example, onepossible solution is

MagSquare=(ReZ/8)*(ReZ/8)+(ImZ/8)*(ImZ/8)

This calculation is performed with a regular multiplication, e.g., ituses an 8-bit by 8-bit multiplication subroutine. The 65536 in the nextequation is a constant to keep the calculations of Y as 16-bit numbers.The units of Y depend on the 65536, the divide by 8 in MagSquare, theinstrumentation amp gain, the ADC resolution, and the applied current.They are chosen to make ReY and ImY span the full range of 16-bit signedintegers.

ReY=(65536*ReZ)/MagSquare

ImY=−(65536*ImZ)/MagSquare

The apparatus 100 is first calibrated using resistors and capacitors ofknown value. One way to perform a phase calibration is to multiply theimpedance signal (or admittance signal) by a complex constant. LetK=e^(jθ) be a complex constant with magnitude 1, and phase θ. When thisis implemented in fixed-point math, it is needed to find three integers{m, n₁, n₂}, where K=(n₁+jn₂)/2^(m), such that K has the desired phase,and the magnitude of K is close to 1. Because the system will becalibrated, the magnitude does not have to be exactly equal to 1. Forexample, K=−1+3j/8 has a magnitude of 1.068 and a phase of 159.7degrees. To correct for 159.7 degrees of phase in the circuit, multiplyZ by K

Z*K=(ReZ+jImZ)*(−1+3j/8)

ReZ _(corrected) =−ReZ−3*ImZ/8

ImZ _(corrected)=3*ReZ/8−ImZ

A second example of this calibration is, K=1−j/4 has a magnitude of 1.03and a phase of 345.96 degrees (−14 degrees). To correct for 345.96degrees of phase, multiply Z by K

Z*K=(ReZ+jImZ)*(1−j/4)

ReZ _(corrected) =ReZ+ImZ/4

ImZ _(corrected) =−ReZ/4+ImZ

The following table is data showing measured ReY and ImY versus trueresistance and conductance. The resistances between lead pins 1-2 and3-4 were fixed at 499Ω each. Six different 1% metal film resistors wereplaced between lead pins 2-3, shown as R₂₋₃ in FIG. 4. The ReZ ImZ ReYand ImY columns show the average of eight repeated measurementscollected by the microcontroller. The standard deviations werecalculated from the eight repeated measurements.

Resistance Conductance (Ω) (μS) ReZ ImZ ReY σ_(ReY) ImY σ_(ImY) |Z| |Y|Phase Y 49.9 20040 198.8 −100.9 19548.5 399.3 3956.6 1413.8 222.920317.3 11.4 100 10000 427.9 −197.1 9105.4 35.1 1720.5 29.1 471.1 9266.510.7 200 5000 883.3 −290.6 4663.0 19.5 340.5 8.5 929.8 4675.4 4.2 3013322 1316.6 −361.6 3174.0 10.0 73.1 2.5 1365.4 3174.8 1.3 383 26111629.6 −415.8 2577.3 9.3 12.6 3.3 1681.8 2577.3 0.3 422 2370 1782.1−448.1 2362.1 6.0 3.5 2.3 1837.6 2362.1 0.1

FIGS. 11a-c show the system is linear when measuring impedance andconductance of 1% metal-film resistors. FIG. 12 and the following tableshow the apparatus 100 is also capable of measuring capacitance(susceptance). The ReY and ImY columns again show the average of eightrepeated measurements collected by the microcontroller. The standarddeviations were calculated from the eight repeated measurements. The“true” values were measured with a digital multimeter.

Resistance Conductance Capacitance Susceptance ReY ImY (Ω) (μS) (pF)(μS) (μS) σ_(ReY) (μS) σ_(ImY) 301 3322 0 0 3274.5 9.7 79.6 3.6 301 3322100 13 3270.6 10.2 94.9 4.6 301 3322 470 59 3275.8 1.8 140.4 4.8 3013322 1000 126 3278.6 9.7 191.9 3.3 301 3322 2200 276 3259.9 11.9 329.64.1 301 3322 4700 591 3248.8 10.3 502.1 5.6 301 3322 10000 1257 3210.99.8 1136.9 5.8

In order to demonstrate how the apparatus 100 can be used to detectheart failure, it was tested with seven precision resistors. Thesimulated LV volume, V, is defined as 2G+40 (this matches experimentaldata obtained in pigs). The “true” resistance values were measured witha 3.5 digit DVM. Admittance was measured 8 times at 10 Hz with a12-cycle chirp, and the standard deviation σ is based on these repeatedmeasurements (FIG. 13).

R G ReY (ohm) (mS) V (mL) (mS) σ_(ReY) ImY YPhase |Y| |Z| 13.17 75.93191.86 75.98 2.13 11.35 8.49 76.82 364.48 15.68 63.78 167.55 62.97 1.217.33 6.64 63.40 435.66 18.94 52.80 145.60 52.99 0.37 4.57 4.92 53.19516.36 23.05 43.38 126.77 43.68 0.36 2.61 3.42 43.76 618.56 26.88 37.20114.40 37.39 0.45 1.65 2.53 37.43 716.19 33.89 29.51 99.01 30.00 0.210.82 1.57 30.01 908.55 50.45 19.82 79.64 20.44 0.11 0.31 0.88 20.441324.50

A saline calibration is used to remove the imaginary part due to thecircuit or the lead. The system will be calibrated in saline to get arelationship between ReY and ImY. Let f be the functional relationshipbetween ImY=f(ReY) in saline, representing the response of the lead.This might be a simple constant, a linear fit or a table lookup withinterpolation. C_(m) is the muscle capacitance, and G_(m) is the muscleconductance. In saline, the calculation of C_(m) should be zero.However, in vivo C_(m) will represent the capacitance of the tissue.

C _(m)=(ImY−f(ReY))/(2π20 kHz)

The constant SigEplRatio is standard a/c ratio

G _(m)=SigEplRatio*C_(m)

G _(b) =ReY−G _(m)

The blood conductance (G_(b)) is used to derive heart volume using theequations developed in prior art.

One option for the apparatus 100 is to implant it in animals. FIG. 2shows the animal system with an admittance-pressure lead in a ventricle.FIG. 5 shows the block diagram of this configuration. The admittancechannel is the same as previously described. This apparatus 100 has apressure sensor (41), amplifier (43) and filter (45). The communicationlink implements wireless communication (52). FIG. 14 shows the wirelessversion of the apparatus 100. The antenna (56) transmits data, and areceiver connected to a computer receives the data. This way,pressure-volume loops are recorded in real time.

There is an alternative low power technique to measure complexelectrical properties using a synchronous demodulator, as shown in FIG.16 [30]. This method can be configured to measure either impedance oradmittance. The method begins with an AC current source (58). Thefrequency of this source will determine the frequency at which theelectrical properties are measured. Similar to the other techniques, thecurrent is applied across electrodes 1 (32) and 4 (38). V₁ (60) is an ACvoltage signal that has an amplitude and phase matching the appliedcurrent. Also similar to the other techniques, the resulting voltageacross electrodes 2 (34) and 3 (36) is measured with an amplifier (42).V_(V) (62) is an AC voltage signal that has an amplitude and phasematching the voltage between electrodes 2 and 3. Because the transduceris excited with a constant current, this signal represents the compleximpedance, Z=ReZ+jImZ. There is a digital signal (66) that controls a2-input 2-output analog switch (64). A peak detector (69) on the signalV_(EX) can be added to assist in calibrating the measurement.

When the apparatus 100 is configured in impedance mode, the analogswitch (64) makes {V₁=V_(V), V_(EX)=V_(I)}. The threshold detector (67)creates a digital square wave, V₀, which is in phase with V_(I). Thephase lock loop, or PLL, (68) creates a digital square wave, V₉₀, whichis 90 degrees out of phase with V_(I). Two four-quadrant mixers (70) areused to separate the real and imaginary parts of impedance Z. Thevoltages V_(real)(72) and V_(imag)(74) are DC signals representing ReZand ImZ respectively.

When the apparatus 100 is configured in admittance mode, the analogswitch (64) makes {V_(EX)=V_(V), V₁=V_(I)}. The threshold detector (67)creates a digital square wave, V₀, which is in phase with V_(V). ThePLL, (68) creates a digital square wave, V₉₀, which is 90 degrees out ofphase with V_(V). In this configuration, the two synchronousdemodulators (70) are used to separate the real and imaginary parts ofadmittance Y. In this mode, the voltages V_(real)(72) and V_(imag)(74)are DC signals representing ReY and ImY respectively.

Because V_(real)(72) and V_(imag)(74) are DC signals, they can besampled with a low power ADC (76) at frequencies determined by thechange in heart volume. For example, if the heart rate is 1 beat persecond, or 60 BPM, 100 volume measurements per beat can be created bysampling the ADC at 100 Hz. The phase correction, calibration anddetermination of heart volume are identical to other techniquesdeveloped by many of the inventors in pending patent applications (seethe list of references below). A board level apparatus 100 was built andcalibrated. The following data shows the technique is capable ofmeasuring complex electrical properties. Although using a synchronousdemodulator has been used previously to measure complex electricalproperties, this apparatus 100 can be easily configured to directlymeasure either impedance or admittance.

R (Ω) C (nF) V_(real) (V) V_(imag) (V) ReZ (Ω) ImZ (Ω) C (nF) 50 00.2127 −0.0142 53.3 0 0 50 10 0.2127 −0.0235 53.5 1233.7 7.2 50 200.2105 −0.0325 53.4 616.2 14.3 50 30 0.2071 −0.0438 53.5 374.6 23.6 1000 0.4225 −0.0268 106.0 0 0 100 10 0.4226 −0.0632 108.0 1256.8 7.0 100 200.4042 −0.097 106.9 606.5 14.6 100 30 0.3872 −0.1333 108.3 386.3 22.9

Example: One embodiment is now presented, which is believed to be thepreferred technique herein at this time to measure heart volumeoptimizing for size and power. The four-electrode lead (32, 34, 36, and38) is placed in either of the two configurations shown in FIGS. 1a and1 b. A 20 kHz sine wave current (30) is applied across electrodes 1 (32)and 4 (38) for 400 μsec, and the resulting voltage measured betweenelectrodes 2 (34) and 3 (36) is amplified (42). The 400 μsec chirp (FIG.3) is sampled by the ADC (46) at 160 kHz and software calculates LVvolume from the digitized samples. Typical inputs to the ADC are shownin FIG. 3. One measurement requires 440 μsec, after which the systementers a low-power sleep mode. In this system, LV volume will bemeasured 50 times each hour. The largest volume measured each hour isassumed to be the end-diastolic volume, which is output using asynchronous serial port of the microcontroller. Therefore, the system ispowered for only 4.4 ms each hour.

In this paragraph, the hardware details will be presented, as shown inFIG. 18. The microcontroller (24) is an MSP430F2012, powered at 3.3 V,running at 16 MHz. This apparatus 100 was chosen because of its lowpower. The system requires 2 k ROM, 128 bytes of RAM, 6 digital outputs,and one analog input. The microcontroller output P2.6 is used to powerthe analog circuit. When P2.6 is low, the analog circuit is off, drawingno current. When P2.6 is high (3.3V), the analog circuit is active. Thesystem uses one 3-bit SinDAC (24), with R2=147 kΩ, R1=205 kΩ, and R0=499kΩ. More bits in the SinDAC would give a better sine wave, but 3 bitsrepresent a good tradeoff between accuracy and size. The importantparameter when designing a SinDAC is the number of output per cycle. Ifone were to increase the number to 16, then the microcontroller wouldneed to run twice as fast, requiring more power to operate. In thissystem there are 8 outputs per cycle, and this was chosen to balancemeasurement accuracy and low power. The digital output pattern is3,6,7,6,3,1,0,1. This means the 3-bit binary outputs from theMSP430F2012 (shown as P1.2, P1.1, P1.0 in FIG. 18) will be the sequence:011, 110, 111, 110, 011, 001, 000, 001, which is output at 160 kHz. Thevalues of R2, R1, R0 were found by minimizing the mean squared errorbetween the desired sine wave shape and V_(out), given the constraintthat the resistors must be E-96 standard values larger than 100 kΩ. Thecapacitor shown in FIG. 3 is optional, and has been left off in thissystem to save space. The OPA330AIDCKT op amp (30) is used to convertthe SinDAC voltage to current. The op amp is chosen because of its lowpower and low noise. The capacitor C1 is 2.2 nF, chosen to pass 20 kHzand reject DC. The 1.225 V reference (created with LM4041A12IDCKR)creates a DC offset in the analog circuits, so the entire analog circuitwill run off one 3.3V power supply. R3 is 25.5 kΩ, and it is used toreduce the AC amplitude out of the SinDAC to 0.3V rms. R4 is 4.99 kΩ,and it is used to set the applied current across electrodes 1 and 4 to60 μA rms. C2 and C3 are both 0.1 μF, and their purpose is to prevent DCcurrent from passing across the electrodes. DC current must be removedso the system will exhibit long-term stability. Again the values of C2and C3 are chosen to pass 20 kHz and block DC. The resistors labeledR₃₋₄, R₂₋₃ and R₂₋₁ in FIG. 19 do not represent resistors in thecircuit, but rather they represent impedances in the heart itself. Inparticular, R₃₋₄ means the heart impedance between electrodes 3 and 4;R₂₋₃ means the heart impedance between electrodes 2 and 3; and R₂₋₁means the heart impedance between electrodes 1 and 2. The heart volumeis derived from the blood component measured in R₂₋₃. C4 and C5 are 0.01μF. Similar to C2 and C3, C4 and C5 are used to pass AC signals andblock DC current. Resistors R6 and R7 are 100 kΩ, and provide 1.225 Voffset to the inputs of the instrumentation amp (42). The AD623ARMinstrumentation amp (42) provides a gain of 101. The AD623 was chosenfor its CMRR at 20 kHz, and its low power. The 101 gain essentiallydefines the impedance range of the system. With the 60 μA rms currentand gain of 101, the measurement range of impedance is 10 to 150Ω. Thisrange allows measurements in humans, dogs and pigs with the eitherelectrode configuration shown in FIGS. 1 a, 1 b. One could add an analoglow pass filter (44) between the instrumentation amp (42) and the ADC(46). The purpose of the filter would be to pass 20 kHz and reject 140and 180 kHz. The measurements are just a little bit noisier without thefilter. So, in this system the filter was not included to save power andspace. The 10-bit ADC is sampled at 160 kHz, synchronized to the 160 kHzdigital output to the SinDAC.

In this paragraph, the software details will be presented, as shown inFIG. 19. Fifty times an hour, the system awakes from deep sleep using atimer built into the MSP430. A counter, n, is set to 2 (78), whichdetermines the length of the chirp length. Next, the ADC and analogcircuits are turned on (80). To save space and lower power, the actualmicrocontroller output P2.6 serves as the analog power input for thethree analog chips, labeled as aVcc in FIG. 18. For each n, there willbe 32 SinDAC outputs and 32 ADC inputs (82). Since there are 8 outputsper cycle in the sine wave, 64 outputs means there will be 8 sine wavecycles. Since the output/input pairs occur at 160 kHz, the chirp lasts400 μsec. The system records the last 32 ADC inputs in the variables x₀to x₃₁. After the data are collected, the ADC and analog circuit ispowered down (83). Next, the 32-point DFT is calculated on collecteddata x₀ to x₃₁ (84).

ReZ=(17*(x ₀ −x ₄ +x ₈ −x ₁₂ +x ₁₆ −x ₂₀ +x ₂₄ −x ₂₈)+12*(x ₁ −x ₃ −x ₅+x ₇ +x ₉ −x ₁₁ −x ₁₃ +x ₁₅ +x ₁₇ −x ₁₉ −x ₂₁ +x ₂₃ +x ₂₅ −x ₂₇ −x ₂₉ +x₃₁))/32

ImZ=(17*(−x ₂ +x ₆ −x ₁₀ +x ₁₄ −x ₁₈ +x ₂₂ −x ₂₆ +x ₃₀)+12*(−x ₁ −x ₃ +x₅ +x ₇ −x ₉ −x ₁₁ +x ₁₃ +x ₁₅ −x ₁₇ −x ₁₉ +x ₂₁ +x ₂₃ −x ₂₅ −x ₂₇ +x ₂₉+x ₃₁))/32

To save power, a microcontroller without hardware multiply/divide wasused. Multiply by constants are implemented with shifts and adds. Forexample, A=17*B is calculated as A=(B<<4)+B. Similarly, A=12*B iscalculated as A=(B<<3)+(B<<2). The divide by 32 is implemented as aright shift. The lead is calibrated in saline, where no imaginary partis expected (i.e., ImZ should be zero). To correct for phase shift inthe circuit and lead, the input is multiplied by a complex constant K(86). This system required a correction of −16 degrees, and uses a Kequal to 7/8−j/4, which has a magnitude of 0.91 and a phase of 344.05.To correct for phase, multiply Z by K

Z*K=(ReZ+jImZ)*(7/8−j/4)

ReZ _(corrected)=7*ReZ/8+ImZ/4

ImZ _(corrected) =−ReZ/4+7*ImZ/8

Again, the multiply by 7 was implemented as a series of shifts and adds.The σ/ε ratio is a muscle property constant, relating muscle resistanceto capacitance.

ε_(m)/ε_(m) =R _(m) *C _(m)

A series R-C model is used to convert ImZ to the impedance of themuscle, R_(m) [31]. ω is equal to 2 times π times 20 kHz.

$R_{m} = \frac{{- {{Im}Z}}*( {1 + ( \frac{{\omega ɛ}_{m}}{\sigma_{m}} )^{2}} )}{\frac{{\omega ɛ}_{m}}{\sigma_{m}}}$

The blood resistance, R_(b), is calculated by subtracting off the musclecomponent (90) [31].

$R_{b} = {{{{Re}Z} - \frac{R_{m}}{1 + ( {\omega \; R_{m}C_{m}} )^{2}}} = {{{Re}Z} + {{{Im}Z}*\frac{\sigma_{m}}{{\omega ɛ}_{m}}}}}$

The software performs this calculation using fixed point math.

R _(b) =ReZ+(c*ImZ)/2^(m)

where c and m are integers, such that c/2^(m) approximates σ/εω. Ifdesired, the blood conductance is inversely related to impedance,G_(b)=1/R_(b). The R_(b) and G_(b) measurements can be used as arelative measure of volume. For example, R_(b) could be used to optimizea pacemaker timing or used to detect impending heart failure.

The relationship between blood resistance R_(b) and volume V (92) isnonlinear. If desired, the apparatus 100 uses either piece-wise linearfit or a parametric equation to quantify volume. This relation iscalibrated in vivo using known volumes.

The maximum volume over the last hour is saved in Vmax (94). Once anhour this measurement is output to other modules in the system using asynchronous serial protocol (96). 49 times out of 50, the active modefunctions require 440 μs. Every 50^(th) activation, the synchronousserial output requires an additional 20 μs.

Power was measured with the apparatus 100 with f_(s) equal to 20 kHz anda chirp rate set at 10 Hz. This system just measured volume (no pressuremeasurements or wireless communication.) When in active mode, the systemrequires an average of 6.6 mA. Active mode runs for 440 μs every 100 mswhile chirping. To save power, all analog electronics is turned offwhile sleeping. In sleep mode with periodic wakeup, the system required0.5 μA. FIGS. 10a and 10b plot the analog power supply voltage showingthe chirp occurring every 100 ms. With 8 cycles/chirp, the analog poweris applied for 400 μs, which is 0.4% of the time. It takes about 40 μsto perform the software calculations, so the apparatus 100 is active0.44% of the time. This is an average of about 45 μA. If a chirp for 5seconds occurs once an hour (50 volume measurements an hour), theaverage current drops to less than 1 μA.

6.6 mA*440 μs/100 ms*50 chirps/hr*1 hr/3600 sec+0.5 μA=0.9 μA

Table 1

Comparison to Prior Art

-   Sunagawa [32] made a portable system for rats using dual frequency-   Raghavan [33-34] made a portable system for rats using admittance-   Best practice, in terms of what has been actually built and is    operative for the technique described herein of the present    invention without wireless or pressure.-   Best practice in terms of what has been actually built and is    operative for the technique described herein of the present    invention with wireless and pressure.

Best + wireless + Sunagawa Raghavan Best pressure Current while 42 mA6.6 mA 5 mA sampling Average current 24 mA 0.9 μA 60 μA Average power =86 mW 3.2 μW 216 μW current*3.6 V Size (wo 11 mL 14 mL 1.0 mL 2.3 mLbattery, lead) Weight (wo 14 g 9 g 1.3 g 2.6 g battery, lead)

The technique applies a sinusoidal current at one specific frequency, f.This current does not stimulate the tissue. The apparatus 100 uses oneor more SinDACs to create sine waves at frequency f. When using multipleSinDACs, the phase between the waves can be precisely controlled. E.g.,a phase of 0, 90, 120, 180, 240, or 270 degrees can easily be achieved.The apparatus 100 uses software to create chirped stimulations,resulting in very low-power operation compared to the now-standardcontinuous wave embodiments. FIG. 3 shows experimental recordings at theADC input made with the apparatus 100, showing the dependence on loadresistance (blood volume) and the chirped sinusoid. Setting thefrequency involves a single software parameter, which is easy to change.The experimental data presented here was obtained at f=20 kHz, but themethod will work for any frequency below 100 kHz and is limited only bythe speed of existing off-the-shelf low-power microcontrollers andanalog electronics. Using the software algorithm with the hardwareSinDAC removes the crystal oscillator and high-Q frequency-select analogfilter required by previous systems.

There is software synchronization of sine wave output generated by theSinDAC and the ADC sampling input. This synchronization provides foraccurate measurements of the phase between current output and voltageinput. DAC outputs and ADC inputs are triggered by the same software andoccur at an integer multiple (M) of f. The accurate measurement of phasemeans the system is capable of distinguishing between the real andimaginary parts of the electrical property measurement. To reduce power,M is selected to be a power of 2. I.e., M=2^(m), where m is a positiveinteger. For example, if M=8, there are 8 DAC outputs and 8 ADC samplesper sinusoid period. However, the approach will work for any integer M(e.g., there is an M=12 example shown later.)

The apparatus 100 calculates one term of an N-point DFT. To reducepower, N is also selected to be a power of 2. I.e., N=2^(n)l , where nis a positive integer greater than or equal to m. This allows for asimple calculation of the real part of impedance and imaginary part ofimpedance at one specific frequency (f), without having to measuremagnitude and phase. Power is saved by performing these calculations insoftware, rather than using analog electronics to create the outputsignal. However, the approach will work for any integer N (e.g., thereis an N=24 example shown earlier in paragraph 107.)

The apparatus 100 measures the real and imaginary parts of impedancedirectly. If needed, the real and imaginary parts of admittance arecalculated by inverting impedance in software. This calculationeliminates the analog divider found in previous systems, thus reducingpower. Furthermore, the apparatus 100 does not actually measuremagnitude or phase, because the real and imaginary parts are sufficientto derive cardiac volumes. This approach removes the rectifier and phasedetection analog hardware found in all previous systems. When detectingheart failure and when optimizing pacemaker timing, only the real andimaginary parts of impedance are needed. When measuring heart volumewith a lead inside the ventricle, the real and imaginary parts ofadmittance are required. The fact that the new embodiment apparatus 100measures the needed parameters directly removes the necessity to performpower-expensive trigonometric calculations (e.g., sine, cosine, andarctan), found in existing apparatuses.

The microcontroller can turn on the analog power for the analogsubsystem to perform the measurement, and it can turn off the power whenthe analog subsystem is not needed. More specifically, a digital outputof the microcontroller serves as the analog power signal. This means theanalog circuit requires no current at all while in sleep mode.Furthermore, the microcontroller can put itself into low-power sleepmode, such that the entire system requires much less than 1 μA whilesleeping. If fifty volume measurements are made every hour, theprototype apparatus 100 will run using a time-average current less than1 μA.

There are two trigger modes to awake the apparatus 100 from sleep.First, the apparatus 100 can be programmed to awake periodically (e.g.,once an hour). Second, a digital input pin (50) can be used to awake thesystem. For example, if there is an ECG, then this trigger can beconfigured to sample heart volume at end diastole.

Although the DFT is a complex algorithm usually requiring significantprocessing power, an implementation has been developed that calculates asingle point of an N-point DFT using extremely modest computerprocessing power. The implementation needs only 16-bit addition,subtraction, and shift operations. If N is chosen to be 32, as oneexample, then the method requires only 128 bytes of RAM. In particular,no hardware support for multiply, divide, or floating point calculationis required. Although the initial prototype was implemented using anMSP430 (see FIGS. 8 and 14), the algorithm can run on anymicrocontroller with digital outputs and an analog input. It can even beimplemented in digital logic. This simplicity makes it much easier toembed into existing pacemaker apparatuses. The software runs in realtime, and the results can be output in serial fashion.

An alternate implementation of the measurement has been built and testedusing a synchronous demodulation technique (FIG. 16). The circuitcreates separate analog signals representing the real part and imaginaryparts of admittance, using two four-quadrant mixers (70) to extract thein-phase and in-quadrature components of the measured signal. Using ananalog switch (64), it can be configured to create analog signalsrepresenting either impedance or admittance. The advantage of thisapproach is the ADC is sampled only twice per measurement. Although thesynchronous demodulation technique is a standard approach forcapacitance measurement, the apparatus 100 operates at low power.

DRAWINGS—REFERENCE NUMERALS

10—Four electrodes for admittance measurement, located in the ventricle

12—A single lead placed in the ventricle

14—The apparatus can be added to or embedded into existing apparatuses,such as a pacemaker

16—The system can communicate with the patient or medical staff througha transmitter

18—Electrodes 1 and 2 placed on the heart (in a coronary vein)

20—Electrodes 3 and 4 placed in the heart (in a ventricle or atrium)

22—Paths as current flows between electrodes 1 and 4

24—Microcontroller or digital logic

26—Digital outputs synchronized (controlled) by software

28—One or more SinDACs used to create a sine wave at a specificfrequency

30—Voltage to current circuit applying current to electrodes 1 and 4

32—Electrode 1, current stimulation

34—Electrode 2, voltage sensing

36—Electrode 3, voltage sensing

38—Electrode 4, current stimulation

40—Resistance R₂₋₃ represents the resistance from the blood volume inthe heart

41—Pressure sensor (optional)

42—Low power amplifier

43—Low power amplifier for the pressure channel (optional)

44—Low power analog filter (optional)

45—Low power analog filter for the pressure channel (optional)

46—ADC with sampling synchronized to SinDAC outputs

48—Software algorithm to measure heart volume from electrical propertymeasurements

50—Possible external trigger input to wake up the apparatus

52—Digital link to other system components, for example a wireless link

54—4-wire connector to electrode

56—Antenna used to send wireless communication

58—AC current source driving electrodes 1 and 4 with a constant current

60—AC voltage (V_(I)) with amplitude and phase related to the appliedcurrent applied to electrodes 1 and 4

62—AC voltage (V_(V)) with amplitude and phase related to the resultingvoltage measured between electrodes 2 and 3

64—Analog switch connecting either {V_(EX)=V_(V), V₁=V_(I)} or{V_(I)=V_(V), V_(EX)=V_(I)}

66—Digital control of the analog switch specifying either impedance oradmittance mode

67—Threshold detector used to create digital signal V₀, which is inphase with V_(EX)

68—Phase lock loop used to create digital signal V₉₀, which is 90 out ofphase from V_(EX)

69—Peak detector

70—Four-quadrant mixers, used to mix the two input signals

72—Voltage (V_(real)) proportional to the real part of the input signal

74—Voltage (V_(imag)) proportional to the imaginary part of the inputsignal

76—Low power ADC

78—The constant 2 defines the length of the chirp, n=2 means 8 sinewaves

80—The software can turn on the analog power by setting P2.6 high

82—Each SinDAC output is synchronized to an ADC input

83—The software can turn off the analog power by clearing P2.6 low

84—Discrete Fourier Transform

86—Phase correction

88—Calculate muscle component

90—Remove muscle component from impedance signal

92—Calculate volume

94—Find the largest volume over the last hour

96—Once an hour output maximum volume using synchronous serial SPI

98—Put the system in deep sleep mode

100—The apparatus includes three parts: the stimulator, the sensor, andthe signal processor

110—The apparatus is placed in a housing that provides protection andpower

112—The simulator injects voltage or current into the body

114—The sensor measures the response to the stimulation

116—Using either or both the stimulation and sensor response, the signalprocessor calculates pressure and/or heart volume

Although the invention has been described in detail in the foregoingembodiments for the purpose of illustration, it is to be understood thatsuch detail is solely for that purpose and that variations can be madetherein by those skilled in the art without departing from the spiritand scope of the invention except as it may be described by thefollowing claims.

Appendix

Literature Cited, all of which are Incorporated by Reference herein.

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1. An apparatus for measuring complex electrical admittance and/orcomplex electrical impedance in animal or human patients comprising: twoor more electrodes that are adapted to be disposed in the patient; and ahousing adapted to be disposed in the patient, the housing havingdisposed in it a stimulator in electrical communication with two or moreelectrodes to stimulate with either current or voltage, a sensor inelectrical communication with the same stimulating electrodes or withadditional electrodes to sense a response based on the stimulation ofthe stimulating electrodes, and a signal processor in electricalcommunication with both the stimulator and the sensor to determine thecomplex electrical admittance or impedance of the patient, thestimulator and the sensor and the signal processor together using lessthan an average current of less than 23 mA in operation over time.
 2. Anapparatus as described in claim 1 wherein a real part, an imaginarypart, a magnitude, and/or phase of admittance can be measured.
 3. Anapparatus described in claim 2 wherein a real part, an imaginary part, amagnitude, and/or phase of impedance can be measured.
 4. An apparatusdescribed in claim 3 wherein the stimulator produces an excitation wavethat is a sinusoid at a single frequency, greater than 0 and less thanor equal to 1 MHz.
 5. An apparatus described in claim 3 wherein thestimulator produces an excitation wave that is two or more sinusoidswith frequencies greater than 0 and less than or equal to 1 MHz.
 6. Anapparatus described in claim 3 wherein the stimulator produces anexcitation wave that is any shape that can be defined by a repeatedsequence of integer values, whose frequency components range from 0 to 1MHz.
 7. An apparatus described in claim 6 wherein the stimulatorproduces an excitation wave that is created by a resistor-summingnetwork, called a SinDAC, such that a number of resistors, resistorvalues, digital output sequence, and rate of digital outputs areselected to define a shape and frequency of the excitation wave.
 8. Anapparatus described in claim 7 wherein an ADC conversion of the sensoris synchronized to the SinDAC outputs generating the stimulation.
 9. Anapparatus described in claim 8 wherein a Discrete Fourier Transform(DFT) is used by the signal processor to extract complex electricalproperties.
 10. An apparatus described in claim 4 wherein the complexmeasurements occurs with an analog circuit using a synchronousdemodulator to directly measure either impedance or admittance.
 11. Anapparatus described in claim 9 or claim 10 wherein while measuringcomplex electrical properties 100 times a second requires less than 500μA of current.
 12. An apparatus described in claim 11 wherein whilemeasuring complex electrical properties 50 times an hour requires lessthan 1 μA of current.
 13. An apparatus described in claim 12 wherein thesize of the housing is less than 2 cm by 2 cm by 0.4 cm.
 14. Anapparatus described in claim 13 wherein the electrodes are placed in oron the heart, which are used to estimate heart volume, stroke volume,change in heart volume, and/or change in stroke volume of the patient.15. An apparatus described in claim 14 including a pressure sensordisposed on the lead, which is used to measure pressure volume loops inthe heart.
 16. An apparatus described in claim 15 including a wirelesslink and recording base station, which are used to remotely measurepressure, heart volume, stroke volume, change in heart volume, and/orchange in stroke volume of the patient.
 17. An apparatus described inclaim 1 wherein the housing includes a pacemaker.
 18. A method formeasuring complex electrical admittance and/or complex electricalimpedance in animal or human patients comprising the steps of:stimulating with a stimulator disposed in a housing disposed in thepatient with two or more electrodes disposed in the patient with eithercurrent or voltage; sensing with a sensor disposed in the housing withtwo or more sensing electrodes disposed in the patient to sense aresponse from the sensing electrodes based on the stimulation of thestimulating electrodes; and determining with a signal processor disposedin the housing and in electrical communication with both the stimulatorand the sensor the complex electrical admittance and/or complexelectrical impedance of the patient, the stimulator and the sensor andthe signal processor together using less than an average current of lessthan 23 mA in operation over time at a voltage less than 3.7 V.
 19. Anapparatus for measuring complex electrical admittance and/or complexelectrical impedance in animal or human patients comprising: a firstelectrode and at least a second electrode that are adapted to bedisposed in the patient; and a housing adapted to be disposed in thepatient, the housing having disposed in it a stimulator in electricalcommunication with at least the first electrode to stimulate the firstelectrode with either current or voltage, a sensor in electricalcommunication with at least the second electrode to sense a responsefrom the second electrode based on the stimulation of the firstelectrode, and a signal processor in electrical communication with thesensor to determine the complex electrical admittance or impedance ofthe patient, the stimulator and the sensor and the signal processortogether using less than an average current of less than 23 mA inoperation over time.
 20. A method for measuring complex electricaladmittance and/or complex electrical impedance in animal or humanpatients comprising the steps of: stimulating with a stimulator disposedin a housing disposed in the patient with at least two stimulatingelectrodes disposed in the patient with either current or voltage;sensing with a sensor disposed in the housing with at least two sensingelectrodes disposed in the patient to sense a response from the sensingelectrodes based on the stimulation of the simulating electrodes; anddetermining with a signal processor disposed in the housing and inelectrical communication with the sensor the complex electricaladmittance or impedance of the patient, the stimulator and the sensorand the signal processor together using less than an average current ofless than 23 mA in operation over time.